The invention relates to a laser processing apparatus in particular a medical laser processing apparatus, for plasma-induced ablation, which comprises a laser light source for generating a processing laser beam, a focussing means for focussing the processing laser beam and an optical detection device for detecting the plasma radiation generated during an ablation.
Such laser processing apparatuses are used in different areas and for different purposes in diverse forms. They are used in particular in the medical field, primarily in dentistry, where they are used instead of a mechanical drill for the ablation or removal of tooth material, in particular carious tooth material.
In contrast to mechanical drill devices, in which a contact between the processing device and the region to be worked is always prescribed for removal purposes, in the case of the laser processing apparatuses which enable contactless processing and removal, the correct positioning or the correct processing distance with respect to the material to be processed is of major importance.
Therefore, it has been proposed in the prior art to equip such laser processing apparatuses, in particular handpieces for such laser processing apparatuses, with a device for distance measurement.
The German patent application (official file reference 100 42 220.9) describes a handpiece for a laser processing apparatus which is provided with a distance measuring device which monitors the distance between a handpiece and the material to be processed. The distance measuring device proposed is a proximity sensor, for example an ultrasonic sensor, for contactlessly measuring the required distance or a mechanical distance pin which is fitted to the handpiece in the vicinity of the region to be processed and to the handpiece preferably in the direction of the emerging laser light.
However, such distance measuring devices are in part complex and costly and in some instances susceptible to faults and make the laser processing apparatus more expensive.
Therefore, it is an object of the present invention to provide a laser processing apparatus which enables a cost-effective and largely fault-unsusceptible ablation of a material and, in particular, satisfies very stringent safety requirements during the operation of the apparatus and precludes endangerment of the patient and/or also of the user or the operator of such a laser processing apparatus or, if appropriate, also of third parties.
This object is achieved by means of a laser processing apparatus according to claim 1; claims 2 to 13 relate to particularly advantageous embodiments of the laser processing apparatus according to the invention.
According to the invention, the laser processing apparatus comprises a laser light source for generating a processing laser beam, a focussing means for focussing the processing laser beam and an optical detection device for detecting the plasma radiation generated during the ablation of the material, the laser processing apparatus furthermore comprising an evaluation and/or a control unit, which are designed in such a way that it automatically switches the laser processing apparatus from a processing mode into a quiescent mode of the laser processing apparatus or of the laser light source and/or vice versa in a manner dependent on the values for the intensity of the plasma radiation which are determined by the detection device.
The invention has the advantage that it is not necessary for the distance to be measured by means of an additional distance measuring device; by contrast, it is possible to ensure that the correct distance or the desired distance range is complied with merely by monitoring the plasma radiation which arises during the ablation or the removal of the material to be processed. By way of example, if the laser processing apparatus is not operated correctly by the operator, e.g. by virtue of a dental handpiece of such a laser processing apparatus being held in such a way that the distance between the handpiece and the material to be processed becomes too large or too small, the excessively low power density or the excessively low fluence at the material to be processed means that no plasma-induced ablation and therefore no plasma radiation arises or the intensity of the plasma decreases at least to a very great extent, so that, from monitoring the plasma radiation alone, it is possible to draw the desired conclusions with regard to the correct distance from the material to be processed.
In the case, moreover, where an operator holds the laser processing apparatus or a corresponding handpiece in such a way that an emitted laser processing beam does not impinge on the material to be processed, which can lead to endangerment of the patient, no plasma radiation is detected and the laser processing apparatus is automatically put into a quiescent mode, thus enabling a secure and hazard-free treatment.
It shall be pointed out at this juncture that the intensity of the plasma can be measured essentially independently of wavelength, in particular can be measured over the entire wavelength range. However, it is also possible for the intensity to be measured only in a specific selected wavelength range or in a plurality of wavelength ranges, which can preferably be chosen depending on the area of application, or else only at one or a plurality of specific wavelengths.
The values determined by the optical detection device are preferably forwarded to the evaluation and/or control unit in the form of a signal, the evaluation unit regarding a sharp drop in the intensity of the plasma radiation, if appropriate a drop down to 0, as an indicator of the fact that the material to be processed is no longer situated in the region of the focus or of a predetermined depth of field range and the desired distances between, for example, a handpiece and the area to be processed or the material to be processed are therefore not being complied with, so that effective or secure processing is longer ensured.
In this case, according to the invention, the evaluation and/or control unit are designed in such a way that they automatically switch the laser processing apparatus or the laser light source back and forth between a processing mode and a quiescent mode in a manner dependent on the values determined by the optical detection device and the corresponding signal generated by the detection device. It can thus be ensured that the laser processing apparatus is switched into a quiescent mode in the event of a processing or treatment in which the required distances are not complied with, which leads to a change in the intensity of the plasma radiation or a complete extinction of the plasma.
The laser processing apparatus according to the invention thus ensures, in a simple and cost-effective manner, that the laser processing apparatus is in a processing mode and a laser processing beam can be emitted only when the correct and desired distances for processing or for treatment are complied with, and an effective and secure processing or treatment is therefore ensured. Therefore, in the case of a laser processing apparatus according to the invention, it is possible to dispense with a distance monitoring device, it being ensured at the same time, however, that the laser processing apparatus or the laser light source does not emit a processing laser beam unless a plasma-induced ablation is also brought about by the latter.
Emission of a processing laser beam without a plasma-induced ablation must be avoided in particular because the energy radiated in by the laser, if it does not lead to the desired ablation, may lead in particular to heating of the material being irradiated, which can lead to undesired damaged to the material or to injuries. The laser processing apparatus according to the invention therefore ensures, in a simple and cost-effective and also a reliable manner, that a laser power is output only when it also actually leads, as desired, to the plasma-induced ablation.
The optical detection device may be a photodetector or else a spectrometer, but it is also possible to provide other sensors which can evaluate the intensity of an illumination, either in a defined wavelength range, in a wavelength-specific manner or in a wavelength-nonspecific manner.
In a preferred embodiment, the evaluation and/or the control unit are designed as an integral component, the evaluation and/or the control unit being realized in particular by a microcomputer. However, it is also possible for evaluation unit and control unit to be mutually separate units or components.
In a particularly preferred embodiment of the laser processing apparatus, the evaluation unit and/or the control unit are designed in such a way that the laser processing apparatus is automatically switched from a processing mode into a quiescent mode if the intensity of the plasma radiation detected by the detection device falls below a fixed first threshold value. This first threshold value is preferably a very low value; in particular, the first threshold value may be 0 or just above a value of 0, so that possible xe2x80x9cnoisexe2x80x9d of the optical detection device and/or of the evaluation unit and/or of the control unit is encompassed. Preferably, a first threshold value is from 10% to 50% above the value of a so-called xe2x80x9cdark measurementxe2x80x9d, i.e. a measurement of the light intensity of the region to be processed without emission of a processing laser beam. Such a dark measurement is preferably carried out automatically by the laser processing apparatus before switching into an operating mode, so that an absolute value for the first threshold value can automatically be defined when prescribing the corresponding relative values (of the first threshold value with regard to the xe2x80x9cdark valuexe2x80x9d).
The relative values for the first threshold value are preferably adjustable; in particular, the first threshold value may also be significantly higher, for example between 100% and 300% above the value of the corresponding dark measurement.
It may be provided that a dark measurement is carried out at regular intervals and the first threshold value is regularly updated as a result; it is also possible for the laser processing apparatus to be designed in such a way that an average value of the last three dark measurements is in each case used as reference value for the first threshold value, that is to say as value for the dark measurement.
In a further preferred embodiment of the laser processing apparatus according to the invention, the evaluation unit and/or the control unit are designed in such a way that the laser processing apparatus can furthermore be operated in a so-called trial mode, preferably in a temporally very narrowly limited trial mode, which may be regarded as part of the quiescent mode or lies within said quiescent mode. During the trial mode, although the laser processing apparatus is not in the actual operating mode, a processing laser beam is emitted on a trial basis and in a temporally restricted manner, in particular in order to check whether the medical laser treatment apparatus or a handpiece or angle member is situated in the correct position for processing. If the processing laser beam emitted during the quiescent mode generates a plasma and if the intensity of this plasma radiation which is detected by the optical detection device at a specific measurement time lies above a definable second threshold value, then the laser processing apparatus is preferably automatically switched into the processing mode. Such a trial mode may be provided when a processing is initially started; however, it is also possible for the laser processing apparatus to run through a trial mode after the laser processing apparatus was switched, during the processing, from the processing mode automatically into a quiescent mode because the detected intensity of the plasma radiation was too low.
In a specific embodiment of the laser processing apparatus according to the invention, it is possible for the second threshold value to correspond to the first threshold value, but the second threshold value preferably lies above the first threshold value (preferably 10% to 30%, if appropriate even 50% to 80%, above the first threshold value), so that a hysteresis is produced.
The laser processing apparatus preferably furthermore comprises an activation device, which can be actuated by a user and is designed in such a way that it puts the laser processing apparatus into the quiescent mode if it is not actuated by the user. It is thus ensured that a processing laser beam is emitted only when the user actively desires processing. Therefore, a processing laser beam is emitted only when, on the one hand, the activation device is actuated by the user and, on the other hand, values for the intensity of the plasma radiation which lie within a previously defined range are supplied by the optical detection device. If one of these conditions is not met, the laser processing apparatus or the laser light source is in a quiescent mode.
In particular when a user-actuable activation device is provided, the laser processing apparatus is preferably designed in such a way that each time the activation device is actuated anew by the user, a dark measurement is automatically carried out first, so that the first and, if appropriate, the second threshold value are automatically defined.
It shall be pointed out at this juncture that the term xe2x80x9cquiescent modexe2x80x9d of the laser processing apparatus or the laser light source can mean, on the one hand, that the laser light source is switched off, in other words no processing laser beam whatsoever is generated; however, it is also possible for the laser light source itself not to be switched off, but for the processing laser beam generated by the laser light source to be shielded, for example by means of a shutter. In the xe2x80x9cprocessing modexe2x80x9d, the processing laser beam is always generated and is not shielded either, so that a laser beam is emitted.
Shielding of the laser light source, for example by means of a shutter, is preferred particularly when a laser light source is used which requires a certain period of time to ensure stable emission of a laser light, this being the case in particular with pulsed laser light sources with short and high-energy laser pulses in the femtoseconds range.
The laser processing apparatus preferably comprises a delay device, which can be designed or set in variable fashion.
The delay device is preferably designed or set in such a way that, in the case where the intensity of the plasma radiation falls below a defined first threshold value (or when switching on the laser processing apparatus or when actuating an activation device), the quiescent mode is switched off only after a defined delay time t1 or after a defined number x of pulses. What is thus made possible is that, in the event of, if appropriate, very short deviations from the desired processing conditions, the processing mode of the laser processing apparatus is maintained, thereby enabling continuous processing if the very short deviations from the desired conditions do not, for example, endanger the patient to be treated.
In a second realization, as an alternative or in addition, the delay device may be designed or set in such a way that, after an automatic changeover of the laser processing apparatus from a processing mode into a quiescent mode, the laser processing apparatus is automatically switched into the processing mode again, at least for a short period of time, after a defined time period t2. What is thus made possible is that the processing is continued or resumed if the desired conditions were not complied with only for a specific period of time, without a user of the laser processing apparatus, for example a treating physician, having to take further measures. However, a processing is continued or resumed only when the activation device is actuated by the user or the laser processing apparatus is actively switched on or actuated by the user through a different measure.
Instead of the defined time t2, it is also possible in this case to define a specific number y of pulses, for example if the laser light sources is not switched off, but rather only shielded.
In a further alternative or additional refinement, it is provided that the delay device is designed or set in such a way that the laser processing apparatus is switched into the operating mode again only for a defined time period t3 or a defined number z of pulses it being changed over into the quiescent mode again after said time t3 or after the z pulses unless the optical detection device forwards to the evaluation and/or control unit an intensity of a plasma radiation or corresponding signals which allow the conclusion to be drawn that the desired conditions are re-established.
If said defined intensity of the plasma radiation is not reached within the time period t3 or within the z pulses generated, a changeover is made to the quiescent mode again.
In a further embodiment, this method can then be carried out anew, for example after a time t2 or after y pulses. In a particular preferred embodiment, however, this method is carried out only over a defined number of repetitions, in which case, by way of example, after a number of 3 to 5 repetitions, an automatic changeover to the processing mode is no longer effected, in particular the processing can only be started again by means of, for example, a deactivation and renewed actuation of the activation device by the user.
In a preferred embodiment, both the threshold values and the fluence of the laser processing beam are adjustable, so that the laser processing apparatus can be set to the desired situation in a flexible manner. The predetermined time periods t1, t2 and t3 preferably lie in a range of between 20 xcexcs and 100 ms, in particular in a range of from 0.1 to 10 ms, preferably in a range of from 1 ms to 5 ms. For the time period t3, in particular, it is preferably the case that very short time periods are defined, in particular in a range below 1 ms. A time period t3xe2x89xa6500 xcexcs is preferably chosen, in particular in a range of between 10 xcexcs and 300 xcexcs, in particular approximately 100 xcexcs.
Instead of the time periods t1 t2 and t3, it is also possible to define a specific number x, y and z of pulses, preferred values for the number x, y and z of pulses lying between 1 and 100, in particular in the range of from 5 to 30. A particularly preferred value is 10 pulses. It is also the case when defining the number of pulses that, in a preferred embodiment, in particular the number z of pulses can be chosen to be very low, a value which does not exceed 10 preferably being chosen for z, in particular z lying between 1 and 5 pulses.
In a particularly preferred embodiment, the laser processing apparatus furthermore comprises a synchronizing device by means of which the optical detection device for detecting the plasma radiation generated during an ablation, on the one hand, and the evaluation and/or control unit and the laser light source on the other hand, are adapted to one another. Such a synchronizing device is of importance particularly in the case of laser light sources which can also emit a pulsed processing laser beam, in which case it shall be pointed out at this juncture that the laser processing apparatus is suitable in particular precisely for such pulsed laser light sources and the system according to the invention exhibits its significant advantages precisely in the case of such pulsed laser light sources. In this connection, it shall also be pointed out again that the provision of the evaluation and/or control unit according to the invention and the control of the laser processing apparatus which is thus made possible is of extreme importance particularly in the case of high-energy pulsed laser systems, since, in the case where laser pulses are radiated at material without igniting a plasma, which, as explained, occurs particularly if a handpiece or a similar element of the laser processing apparatus is positioned incorrectly, the high-energy laser pulses can penetrate to an undesirable depth, for example into a tooth material, where they may lead to heating of the material. Thus, by way of example, if ignition of a plasma does not take place, such a laser pulse may penetrate several millimeters into the hard tooth substance, as a result of which the pulp can be irreversibly damaged by heating.
Such xe2x80x9cincorrect operationxe2x80x9d of the apparatus is automatically avoided as a result of the laser processing apparatus being put automatically into the quiescent mode, thereby ensuring reliable and safe treatment with such high-energy laser systems.
Since the plasma generated by the ablation also xe2x80x9cpulsatesxe2x80x9d in the case of a pulsed processing laser beam, the points in time or time ranges at which the plasma can actually only occur must also be taken into account, the synchronizing device performing this coordination. The pulsating plasma radiation that is likewise produced, also called a plasma torch pulsates essentially with a repetition rate which corresponds to a pulse repetition rate f of the processing laser beam or the laser light source, the plasma radiation being generated for different lengths of time depending on the pulse duration t of the pulsating processing laser beam.
Therefore, between the emitted pulses of the processing laser beam, so-called xe2x80x9cdark pausesxe2x80x9d occur in which no plasma light can occur since no laser energy is being radiated either. The synchronizing device therefore coordinates the laser processing apparatus with regard to the pulse repetition rate f and, if appropriate, also to the pulse duration t of the laser light source or the processing laser beam, so that the pulse repetition rate f and the pulse duration t are used as characteristic quantities for the synchronizing device and thus for the control and coordination of the laser processing apparatus.
In a preferred embodiment, the pulse repetition frequency f and the pulse duration t and/or the temporal profile of the pulses of the synchronizing device are provided directly by the laser light source, including a so-called xe2x80x9cstart pointxe2x80x9d, that is to say a first occurrence or starting of pulsed operation. In a further embodiment, however, it is also possible for the first occurrence of a plasma event to be chosen as the start point for the time measurement; furthermore, it is also possible for the first two or three plasma events to be used in order automatically to determine not only the start point but also the pulse durations t and the pulse repetition frequency or the repetition rate f. It is also possible for specific characteristic values of the laser light source to be stored and used with the first plasma events for the further coordination of the synchronizing device and the laser processing apparatus.
In a further embodiment, it is possible for each emitted pulse to be tapped off at the laser light source and used as a trigger for the optical detection device, thereby making it possible to register the occurrence of plasma on account of a pulse in real time.
In this way, it is possible to ensure in a simple and cost-effective manner that correct monitoring and control of the operation is possible even in the case of a laser processing apparatus which can generate a pulsed processing laser beam.
It shall be pointed out at this juncture that it is possible, in principle, for the plasma radiation generated to be measured essentially over the entire pulse period xcex94t=1/f, in which case, of course, the time ranges in which no plasma can be produced for lack of energy being radiated in are not taken into account in the signal generation and for the control of the laser processing apparatus, or are correspondingly concomitantly taken into account in the case of a determination of an average value for the detected intensity over a specific measurement time period, which is likewise possible in the context of the invention, or the threshold value or values for the average value of the intensity have to be correspondingly lowered in a manner dependent on the so-called xe2x80x9cdark phasesxe2x80x9d. In a particular embodiment, however, the plasma radiation generated is taken into account only in a time period which is shorter than the pulse period xcex94t, in particular the time periods in which the plasma torch is generated or in which the plasma torch is extinguished and in which, therefore, particular intensity fluctuations occur not being taken into account. In a particular embodiment, the plasma radiation generated is only measured over a time period xcfx84 which is shorter than the pulse period xcex94t, where xcfx84, depending on the pulse duration t, may preferably be between 50 xcexcs and 100 xcexcs, preferably above 50 ps up to 100 ns, but the pulse period xcex94t, depending on the repetition rate f, may in this case preferably lie between 20 xcexcs and 1 ms, in a further preferred embodiment between 2 xcexcs and 50 xcexcs, preferably below 35 xcexcs. In the case of ultrashort laser pulses, in particular, the pulse duration t of the processing laser beam is generally shorter than the total lifetime of the plasma produced by the radiated-in energy; in this case, the plasma may even have a total duration which is up to 100 times or even longer than the pulse duration itself.
In one embodiment, the optical detection device detects the generated plasma radiation only during the desired time ranges; however, it is preferred for the optical detection device to continuously monitor the intensity of a plasma radiation that is possibly generated, but for the intensities determined by the optical detection device to be used only in the desired time periods for the further control of the laser processing apparatus.
Preferably, the laser processing apparatus furthermore comprises a device for generating a pilot laser beam, which may serve, in particular, for positioning a handpiece of the laser processing apparatus or a corresponding device. The pilot laser preferably generates a visible laser beam; in particular, semiconductor diodes are preferably used for generating a pilot laser beam.
In a particularly preferred embodiment, the device for generating a pilot laser beam is designed in such a way that the pilot laser beam has different emission wavelengths, so that the pilot laser beam can be changed over from red to green, for example.
It is preferred for the laser processing apparatus furthermore to comprise a device for optical, acoustic and/or tactile indication of the mode of the laser processing apparatus, so that additional information can be given to the operator, in particular a treating physician, during the treatment. Such a device may, in particular, be coupled to the abovementioned device for generating a pilot laser beam or be realized by said device, in which case, by way of example, a pilot laser beam of a first wavelength (for example in the red region) is emitted if the laser processing apparatus is in the quiescent mode, and a pilot laser beam of a second wavelength (for example in the green region) is emitted if the laser processing apparatus is in a processing mode.
It goes without saying that all other wavelengths are also conceivable in addition to the wavelengths in the red and green wavelength ranges mentioned above merely by way of example. It is preferred for the wavelength for the pilot laser beam to be chosen such that it as far as possible does not influence the measurement of the intensity of the plasma radiation. Conversely, it is also possible for the wavelengths or the wavelength range or ranges used for the measurement of the intensity of the plasma to be correspondingly adapted to the wavelength or wavelengths of the pilot laser beam. By way of example, it is possible to select only those wavelength ranges for the measurement of the intensity which lie outside the wavelength of the pilot laser beam. Since the wavelength of a pilot laser beam is limited to a very great extent, it is also possible to measure the intensity of the plasma radiation essentially over the entire wavelength range, but merely not to take account of the narrow wavelength range in which the pilot laser or lasers radiates or radiate for the measurement. This can also be done, for example, simply by placing a wavelength filter upstream of the optical detection device, which filter filters out precisely the wavelength of the pilot laser beam.
In the case of a laser light source which generates a pulsed laser light, typical pulse repetition rates f typically lie in a range of between 1 kHz and 50 kHz, while typical pulse durations t lie in the picoseconds or femtoseconds range.
If the laser light source operates with a pulse repetition rate f, then the optical detection device can detect a plasma radiation in each case at points in time or time ranges with an interval xcex94tmeas=1/f. In this connection, it shall again be pointed out that the plasma lifetime may lie between 100 ps and 1 ns in the case of laser pulses xe2x89xa61 ps. If the plasma detector is triggered in response to an output signal of the laser, then the laser processing apparatus can be put into the quiescent mode after the failure of just a single plasma torch to appear. In this case, the time interval xcex94tmeas is between 10 s and 1 xcexcs, in particular 20 xcexcs given a repetition rate of 50 kHz, in particular 1 ms given a repetition rate of 1 kHz, in particular 33 xcexcs given a repetition rate of 30 kHz, in particular 100 xcexcs given a repetition rate of 10 kHz, and in particular 67 xcexcs given a repetition rate of 15 kHz.
As has already been indicated in the handwritten documents, it shall be pointed out at this juncture that although it is preferred, as described above, for the optical detection device to measure in each case at points in time or time ranges with an interval which is directly coordinated with the pulse repetition frequency of a pulsed laser light source, it is also possible for the time interval xcex94tmeas to be greater than 1/f and for the interval between two points in time or time ranges to be greater than the interval between two successive pulses. In particular, the interval may amount to a multiple of the intervals of the laser pulses, so that only every second, third or, to put it more generally, every nth pulse is taken into account. This makes it possible also to use inexpensive control systems that are not as fast. A further possibility for adapting the time interval xcex94tmeas to a pulsed laser light source with a pulse repetition frequency f would therefore be in accordance with the formula xcex94tmeas=a*1/f, where a is a natural number.
A detection repetition rate or control repetition rate f, may therefore correspond to the pulse repetition rate f, but may also differ from the pulse repetition rate f; in particular, the pulse repetition rate f may be a multiple of the detection repetition rate fxe2x80x2, so that f=a*fxe2x80x2 holds true.
Consequently, for the evaluation, in the evaluation unit, in addition to the detected brightness value, the temporal sequence of the arriving signals is also compared with the pulse repetition rate f tapped off at the laser, in which case, however, the start point for the measurement does not have to coincide with the start point for the pulse, but rather can be chosen essentially freely in the pulse interval xcex94t. In this case, the start point for the measurement is preferably chosen such that the plasma already formed is optimally detected. The start point for the measurement preferably lies in the time period in which a laser pulse impinges on the material to be processed, or shortly afterward, that is to say while a plasma still exists, but it is also possible, in principle, for the start point for the measurement to lie within a so-called dark phase. It is thus possible to ascertain in an extremely simple manner whether or not a plasma has actually been ignited per pulse.
In a further preferred embodiment, it is also possible to choose the first occurrence of a plasma event as the start point of the time measurement and to interrogate the pulse pauses by means of a table stored in the evaluation unit and to compare them with the brightness values occurring after these times. In yet another preferred embodiment, each emitted pulse is tapped off at the laser and used as a trigger for the brightness measurement, thereby making it possible to register the occurrence of plasma on account of a pulse in real time.
In accordance with a further aspect, the laser processing apparatus comprises a device for generating a distance indicator as optical auxiliary means for the user. Such a device for generating a distance indicator is not comparable to range finding since merely the fact of whether or not the user is at the correct distance is indicated or specified to said user, for example by means of optical projections. Such a device for generating a distance indicator serves, in particular, for giving the user of the laser processing apparatus further expedient information for the handling of, for example, a handpiece of a laser treatment apparatus.
Such a device for generating a distance indicator preferably comprises a projection device for projecting a crosshair in the focal region of the processing laser beam, in which case, in the event of a deviation in the processing distance, i.e. in the case where the focus of the processing laser beam does not lie at the level of the material to be processed, the bars of the crosshair move or shift apart on account of the projection.
Such a projection device may also be combined with the abovementioned device for generating a pilot laser beam and/or a device for the optical, acoustic and/or tactile indication of the mode of the laser processing apparatus.